U.S. patent number 5,257,401 [Application Number 07/507,469] was granted by the patent office on 1993-10-26 for method of maintaining an established connection in a mobile radio system comprising both analog and digital radio channels.
This patent grant is currently assigned to Telefonaktiebolaget L M Ericsson. Invention is credited to Jan E. .ANG.ke S. Dahlin, Jan E. Uddenfeldt.
United States Patent |
5,257,401 |
Dahlin , et al. |
October 26, 1993 |
Method of maintaining an established connection in a mobile radio
system comprising both analog and digital radio channels
Abstract
The invention relates to a method in a mobile radio system
having both digital and analog radio channels. An object of the
invention is that a connection established on a digital radio
channel often shall be possible to maintain even when the time
dispersion on available digital radio channels exceeds the designed
maximum time dispersion of equalizers in receiving stations.
According the invention equalizers in the receivers of the system
may be designed for a smaller maximum time dispersion than the
total time dispersion the whole system shall be capable to handle.
Instead handoff is made to an other channel when the time
dispersion of a used digital channel tends to become too big. When
an other digital radio channel with low time dispersion is
available handoff is preferably done to this channel. Alternatively
or in the absence of such a digital radio channel handoff is
performed to an analog radio channel.
Inventors: |
Dahlin; Jan E. .ANG.ke S.
(Jarfalla, SE), Uddenfeldt; Jan E. (Vallingby,
SE) |
Assignee: |
Telefonaktiebolaget L M
Ericsson (Stockholm, SE)
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Family
ID: |
27355492 |
Appl.
No.: |
07/507,469 |
Filed: |
April 11, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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371126 |
Jun 26, 1989 |
5042082 |
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Foreign Application Priority Data
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Apr 17, 1989 [SE] |
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8901385-8 |
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Current U.S.
Class: |
455/436;
455/553.1; 455/63.1; 375/216 |
Current CPC
Class: |
H04W
36/00837 (20180801); H04L 1/20 (20130101); H04W
36/0066 (20130101); H04L 25/0216 (20130101); H04L
1/208 (20130101); H04W 36/0085 (20180801) |
Current International
Class: |
H04L
25/02 (20060101); H04L 1/20 (20060101); H04Q
7/38 (20060101); H04Q 007/00 () |
Field of
Search: |
;455/33,34,38,44,56,67,185,33.2,34.1,38.1,56.1,676,185.1,63
;375/5,11,12,14,101 ;370/95.1 ;379/59,60,63 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0219559 |
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Apr 1987 |
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EP |
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0318033 |
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May 1989 |
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EP |
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9014730 |
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Nov 1990 |
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EP |
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Other References
A Baier et al., "Bit Synchronization and Timing Sensitivity in
Adaptive Viterbi Equalizers for Narrowband-TDMA Digital Mobile
Radio Systems", 38th IEEE Vehicle Technology Conference, Jun.
15-17, 1988..
|
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Urban; Edward
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Parent Case Text
This is a continuation-in-part of application Ser. No. 371,126,
filed Jun. 26, 1989, now U.S. Pat. No. 5,042,082.
Claims
We claim:
1. In a multi-channel mobile radio system comprising a base station
and a mobile station designed both for digital transmission of
information using digitally modulated radio signals on a digital
radio channel and for analog transmission of information using
analog-modulated radio signals on an analog radio channel, which
mobile station comprises an equalizer designed for a particular
maximum time dispersion of digitally modulated radio signals, a
method of maintaining a connection started on the digital radio
channel comprising the steps of:
estimating time dispersion of the digital radio channel used by the
mobile radio station for the connection; and
comparing the estimated time dispersion with criteria for handoff
to an available analog radio channel and when the estimated time
dispersion corresponds to the criteria for handoff, transmitting to
the mobile station information on the available analog radio
channel and performing handoff of the connection to the available
analog radio channel.
2. A method according to claim 1, comprising the further steps
of:
estimating time dispersion of a possible digital radio channel
available to the mobile station during the time the connection
continues on the analog radio channel; and
comparing the estimated time dispersion of the possible digital
radio channel with criteria for handoff to the possible digital
radio channel and when the estimated time dispersion corresponds to
the criteria for handoff to the possible digital radio channel,
transmitting to the mobile station information on the possible
digital radio channel and performing handoff of the connection to
the possible digital radio channel,
3. The method of claim 2 wherein the digital radio channel, the
analog radio channels and the possible digital radio channel are of
different frequencies.
4. The method of claim 1 wherein the digital radio channel and the
analog radio channels are of different frequencies.
5. In a multi-channel mobile radio system comprising a base station
and a mobile station designed both for digital transmission of
information using digitally modulated radio signals on a digital
radio channel and for analog transmission of information using
analog-modulated radio signals on an analog radio channel, which
mobile station comprises an equalizer designed for a particular
maximum time dispersion of digitally modulated radio signals, a
method of maintaining a connection comprising the steps of:
estimating radio propagation properties of the digital radio
channel used by the mobile radio station for an ongoing connection;
and
comparing the estimated radio propagation properties with criteria
for handoff from the digital radio channel and when the estimated
radio propagation properties correspond to the criteria for
handoff, transmitting to the mobile station information on an
available analog radio channel and performing handoff of the
connection to the available analog radio channel.
6. A method according to claim 5, comprising the further steps
of:
estimating radio propagation properties of a possible digital radio
channel available to the mobile station during the time the
connection continues on the analog radio channel; and
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to the
possible digital radio channel and when the estimated radio
propagation properties of the possible digital radio channel
correspond to the criteria for handoff to the possible digital
radio channel, transmitting to the mobile station information on
the possible digital radio channel and performing handoff of the
connection to the possible digital radio channel.
7. The method of claim 6 wherein the digital radio channel, the
analog radio channels and the possible digital radio channel are of
different frequencies.
8. The method of claim 5 wherein the digital radio channel and the
analog radio channels are of different frequencies.
9. In a multi-channel mobile radio system comprising a base station
and a mobile station designed both for digital transmission of
information using digitally modulated radio signals on a digital
radio channel and for analog transmission of information using
analog-modulated radio signals on an analog radio channel, which
mobile station comprises an equalizer designed for a particular
maximum time dispersion of digitally modulated radio signals, a
method of maintaining a connection comprising the steps of:
estimating radio propagation properties of a first digital radio
channel used by the mobile radio station for an ongoing
connection;
estimating radio propagation properties of a second digital radio
channel available to the mobile station;
comparing the estimated radio propagation properties of the first
and second digital radio channels and when the estimated radio
propagation properties of the first and second digital radio
channels correspond to criteria for handoff to the second digital
radio channel, transmitting to the mobile station information on
the second digital radio channel and performing handoff of the
connection to the second digital radio channel; and
when the estimated radio propagation properties of the first and
second digital radio channels do not correspond to the criteria for
handoff to the second digital radio channel, estimating the radio
propagation properties of an analog radio channel available to the
mobile radio station and comparing the estimated radio propagation
properties of the first digital radio channel and the analog radio
channel with criteria for handoff to the analog radio channel, and
when the estimated radio propagation properties of the first
digital radio channel and the analog radio channel correspond to
criteria for handoff to the analog radio channel, transmitting to
the mobile station information on the analog radio channel and
performing handoff of the connection to the analog radio
channel.
10. A method according to claim 9, comprising the further steps
of:
estimating the radio propagation properties of a possible digital
radio channel available to the mobile station during the time the
connection continues on the analog radio channel; and
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to the
possible digital radio channel and when the estimated radio
propagation properties of the possible digital radio channel
correspond to the criteria for handoff to the possible digital
radio channel, transmitting to the mobile station information on
the possible digital radio channel and performing handoff of the
connection to the possible digital radio channel.
11. The method of claim 10 wherein the first digital radio channel,
the second digital radio channel, the analog radio channels and the
possible digital radio channel are of different frequencies.
12. The method of claim 9 wherein the first digital radio channel,
the second digital radio channel and the analog radio channels are
of different frequencies.
13. In a multiple channel mobile radio system comprising a base
station and a mobile station designed both for digital transmission
of information using digitally modulated radio signals on any of a
plurality of digital radio channels and for analog transmission of
information using analog-modulated radio signals on any of a
plurality of analog radio channels, which mobile station comprises
an equalizer designed for a particular maximum time dispersion of
digitally modulated radio signals, a method of maintaining a
connection comprising the steps of:
estimating a signal level and bit error rate of a first digital
radio channel used by the mobile radio station for an ongoing
connection;
estimating radio propagation properties of a second digital radio
channel available to the mobile station;
comparing the estimated signal level and bit error rate of the
first digital radio channel and the estimated radio propagation
properties of the second digital radio channel and, when the
estimated signal level and bit error rate of the first digital
radio channel and the estimated radio propagation properties of the
second digital radio channel correspond to criteria for handoff to
the second digital radio channel, transmitting to the mobile
station information on the second digital radio channel and
performing handoff of the connection to the second digital radio
channel; and
when the estimated signal level and bit error rate of the first
digital radio channel and the estimated radio propagation
properties of the second digital radio channel do not correspond to
the criteria for handoff to the second digital radio channel,
estimating the radio propagation properties of an analog radio
channel available to the mobile radio station and comparing the
estimated radio propagation properties of the first digital radio
channel and the analog radio channel with criteria for handoff to
the analog radio channel and when the estimated radio propagation
properties of the first digital radio channel and the analog radio
channel correspond to criteria for handoff to the analog radio
channel, transmitting to the mobile station information on the
analog radio channel and performing handoff of the connection to
the analog radio channel.
14. A method according to claim 13, comprising the further steps
of:
estimating radio propagation properties of a possible digital radio
channel available to the mobile station during the time the
connection continues on the analog radio channel;
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to the
possible digital radio channel and when the estimated radio
propagation properties of the possible digital radio channel
correspond to the criteria for handoff to the possible digital
radio channel, transmitting to the mobile station information on
the possible digital radio channel and performing handoff of the
connection to the possible digital radio channel.
15. The method of claim 14 wherein the first digital radio channel,
the second digital radio channel, the analog radio channel and the
possible digital radio channel are of different frequencies.
16. The method of claim 13 wherein the first digital radio channel,
the second digital radio channel and the analog radio channel are
of different frequencies.
17. In a mobile radio system having a mobile station and a base
station having use of analog radio channels and digital radio
channels subject to time dispersion whereby signal intelligence is
smeared over a wider time interval during the course of
transmission, a method of maintaining a connection established on a
digital radio channel, comprising the steps of:
determining when time dispersion on said digital radio channel
exceeds a predetermined maximum acceptable time dispersion; and
transmitting to the mobile station, when the time dispersion of
said digital radio channel exceeds the predetermined maximum
acceptable time dispersion, information on an available analog
channel, and performing handoff of the connection to the available
analog channel.
18. The method of claim 17 wherein said determining step comprises
estimating said time dispersion of said digital radio channel at
said mobile station and reporting estimates to said base
station.
19. The method of claim 17 wherein said digital radio channel is an
uplink channel and said determining step comprises the base station
estimating said time dispersion of said digital radio channel.
20. The method of claim 17 wherein the digital radio channel and
the analog radio channel are of different frequencies.
21. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital transmission
of information using digitally modulated radio signals on a digital
radio channel and for analog transmission of information using
analog modulated radio signals on an analog radio channel, which
mobile station comprises an equalizer designed for a particular
maximum time dispersion of digitally modulated radio signals, a
method of maintaining a connection started on the digital radio
channel comprising the steps of:
estimating time dispersion of the digital radio channel used by the
mobile radio station for the connection;
estimating radio propagation of an analog radio channel available
to the mobile station; and
comparing the estimated time dispersion and radio propagation
properties with criteria for handoff to the available analog radio
channel and when the estimated time dispersion and radio
propagation properties correspond to the criteria for handoff,
transmitting to the mobile station information on the available
analog radio channel and performing handoff of the connection to
the available analog radio channel.
22. The method of claim 21 wherein the digital radio channel and
the analog radio channels are of different frequencies.
23. A method according to claim 21, comprising the further steps
of:
estimating time dispersion of a possible digital radio channel
available to the mobile station during the time the connection
continues on the analog radio channel; and
comparing the estimated time dispersion of the possible digital
radio channel with criteria for handoff to the possible digital
radio channel and when the estimated time dispersion of the
possible digital radio channel substantially corresponds to the
criteria for handoff to the possible digital radio channel,
transmitting to the mobile station information on the possible
digital radio channel and performing handoff of the connection to
the possible digital radio channel.
24. The method of claim 23 wherein the digital radio channel, the
analog radio channels and the possible digital radio channel are of
different frequencies.
25. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital transmission
of information using digitally modulated radio signals on a digital
radio channel and for analog transmission of information using
analog-modulated radio signals on an analog radio channel, which
mobile station comprises an equalizer designed for a particular
maximum time dispersion of digitally modulated radio signals, a
method of maintaining a connection comprising the steps of:
estimating radio propagation properties of the digital radio
channel used by the mobile radio station for an ongoing
connection;
estimating radio propagation properties of an analog radio channel
available to the mobile radio station; and
comparing the estimated radio propagation properties of the digital
radio channel and the available analog radio channel with criteria
for handoff from the digital radio channel to the available analog
radio channel and when the estimated radio propagation properties
of the digital radio channel and the available analog radio channel
correspond to the criteria for handoff, transmitting to the mobile
station information on the available analog radio channel and
performing handoff of the connection to the available analog radio
channel.
26. The method of claim 25 wherein the digital radio channel and
the analog radio channels are of different frequencies.
27. A method according to claim 25, comprising the further steps
of:
estimating radio propagation properties of a possible digital radio
channel available to the mobile station during the time the
connection continues on the analog radio channel; and
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to the
possible digital radio channel and when the estimated radio
propagation properties of the possible digital radio channel
correspond to the criteria for handoff to the possible digital
radio channel, transmitting to the mobile station information on
the possible digital radio channel and performing handoff of the
connection to the possible digital radio channel.
28. The method of claim 27 wherein the digital radio channel, the
analog radio channels and the possible digital radio channel are of
different frequencies.
29. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital transmission
of information using digitally modulated radio signals on a digital
radio channel and for analog transmission of information using
analog-modulated radio signals on an analog radio channel, which
mobile station comprises an equalizer designed for a particular
maximum time dispersion of digitally modulated radio signals, a
method of maintaining a connection comprising the steps of:
estimating ratio propagation properties of a first digital radio
channel used by the mobile radio station for an ongoing
connection;
estimating radio propagation properties of a second digital radio
channel available to the mobile station;
comparing the estimated radio propagation properties of the first
and second digital radio channels and when the estimated radio
propagation properties of the first and second digital radio
channels correspond to criteria for handoff to the second digital
radio channel, transmitting to the mobile station information on
the second digital radio channel and performing handoff of the
connection to the second digital radio channel; and
when the estimated radio propagation properties of the first and
second digital radio channels do not correspond to the criteria for
handoff to the second digital radio channel, transmitting to the
mobile station information on an available analog radio channel and
performing handoff of the connection to the analog radio
channel.
30. The method of claim 29 wherein the first digital radio channel,
the second digital radio channel and the analog radio channels are
of different frequencies.
31. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital transmission
of information using digitally modulated radio signals on any of a
plurality of digital radio channels and for analog transmission of
information using analog-modulated radio signals on any of a
plurality of analog radio channels, which mobile station comprises
an equalizer designed for a particular maximum time dispersion of
digitally modulated radio signals, a method of maintaining a
connection comprising the steps of:
estimating a signal level and bit error rate of a first digital
radio channel used by the mobile radio station for an ongoing
connection;
estimating the radio propagation properties of a second digital
radio channel available to the mobile station;
comparing the estimated signal level and bit error rate of the
first digital radio channel and the estimated radio propagation
properties of the second digital radio channel and when the
estimated signal level and bit error rate of the first digital
radio channel and the estimated radio propagation properties of the
second digital radio channel correspond to criteria for handoff to
the second digital radio channel, transmitting to the mobile
station information on the second digital radio channel and
performing handoff of the connection to the second digital radio
channel; and
when the estimated signal level and bit error rate of the first
digital radio channel and the estimated radio propagation
properties of the second digital radio channel do not correspond to
the criteria for handoff to the second digital radio channel,
transmitting to the mobile station information on an available
analog radio channel and performing handoff of the connection to
the analog radio channel.
32. The method of claim 31 wherein the first digital radio channel,
the second digital radio channel and the analog radio channel are
of different frequencies.
Description
FIELD OF INVENTION
The present invention relates to the technical field of mobile
radio systems. More specifically, the invention relates to a method
in a mobile radio system having both analog and digital radio
channels. The purpose of the invention is that a connection which
has begun on a digital radio channel often shall be possible to
sustain even when the dispersions on available digital radio
channels exceed the maximum time dispersion for which the
equalizers in the mobile stations have been designed. Due to its
character the method might be referred to the technical field of
handover in mobile radio systems.
BACKGROUND OF THE INVENTION
The mobile radio systems first in common use had analog
transmission of information between base stations and mobile
stations, i.e. analog information was transmitted between base and
mobile stations by using analogouslymodulated radio signals on
analog radio channels. By frequency multiplexing technique analog
mobile radio systems may have plural radio channels. The Nordic
mobile telephone system, NMT, is an example of such an analog
mobile radio system having many radio channels.
Recently there has been suggested mobile radio systems with digital
transmission of information between base stations and mobile
stations, i.e. digital or digitized information is transmitted
between base and mobile stations using digitally modulated radio
signals on digital radio channels. Of course more radio channels
may be created by frequency multiplex in digital mobile radio
systems also. In order to get even more channels in digital mobile
radio systems it has been proposed that digital radio channels
shall share a radio frequency in time multiplex. The pan european
mobile radio system, GSM, may be mentioned as an example of such a
digital mobile radio system.
The digital technique means substantial advantages in mobile radio
systems. Accordingly there is an interest in introducing the
digital technique. On the other hand there are in certain areas
existing analog mobile radio systems using the frequency band
available for a digital system. When huge investments have been
made in existing analog systems it is not always reasonable to
immediately replace an existing analog system with a digital
system. It has therefore been suggested to successively replace
analog channels in analog systems having many analog channels in
frequency multiplex by a number of digital channels in time
multiplex. For the United States it has been suggested that each
analog channel shall be replaced by three digital channels in time
multiplex. During a long transitory period of time such a mobile
radio system would then comprise both analog radio channels for
analog transmission of information by using analog-modulated radio
signals and digital radio channels for digital transmission of
information by using digitally modulated radio signals. Mobile
stations which during the transitory time wish to be able to use
the full capacity of such a system must then be able to use both
analog radio channels and digital radio channels.
In digital mobile radio systems problems occur with time dispersion
on the digital radio channels due to reflections and multipath
propagation unless particular measures are taken. The time
dispersion changes from one place to the other due to the radio
propagation properties. (Time dispersion problems have long been
known in the fields of radio and television. For example,
"ghosting" on a television screen is a result of time dispersion
problems associated with receiving a broadcast signal and, after a
time delay, receiving one or more reflections of that signal. In
digital mobile radio systems, a plurality of "ghosts" may occur:
some weak and some quite strong. Equalizers are used to interpret
the broadcast signal and signals resulting from reflections or
multipath propagation to reconstruct the transmitted signal.
Equalizers must interpret these signals where the transmitted
signal and its "ghosts" are of substantially the same signal
strength, are of differing signal strengths, have a low time
dispersion or have a severe time dispersion. Of course, high signal
strength may be associated with low or severe time dispersion as
may low signal strength.) In order to obtain a reliable
transmission of information in spite of time dispersion it is well
known to have equalizers at the receiving side in a mobile radio
system. Depending upon design such an equalizer can handle shorter
or longer time dispersion. One way to describe the ability of an
equalizer to cope with time dispersion is to state the maximum time
dispersion in microseconds the equalizer is designed for. Another
way is to state how many symbol time spans the equalizer can cope
with. The complexity, cost and power consumption of an equalizer
increases progressively with the maximum time dispersion in
microseconds and the number of symbol duration times. Accordingly
there is an interest in designing a mobile radio system such that
the requirements on the equalizers do not become too great and in
particular not become greater than necessary. For the above
mentioned pan european system GSM it has been decided that the
equalizers shall be designed to cope with a maximum time dispersion
of 16 microseconds, which corresponds to four symbol time
intervals. In the above mentioned mobile radio system for the
United States having both digital and analog radio channels the
system should probably need to cope with a maximum time dispersion
of about 60 microseconds on the digital radio channels, which is
more than the duration of one symbol but less than the duration of
two symbols.
The analog transmission of information on the analog radio channels
of existing analog mobile radio systems, e.g. the above mentioned
NMT or of the type existing in the USA, is normally substantially
less sensitive than digital systems to reflections and multipath
propagation causing dispersion on the digital radio channels.
BRIEF DESCRIPTION OF THE INVENTION
The complexity, cost and current consumption of equalizers increase
progressively with the maximum time dispersion that an equalizer
shall be able to handle or the number of symbol duration times the
equalizer shall handle. This causes problems when designing mobile
radio systems where a long time dispersion must be considered. The
problems become particularly great when for various reasons it is
not possible to choose certain parameters more or less freely, e.g.
when desiring to successively change from analog technique to
digital technique in a mobile radio system by successively
replacing analog radio channels by time multiplexed digital radio
channels. It can become particularly unfavorable if the time
dispersion the system ought to successfully handle is substantially
greater than a certain integer number of symbol time intervals but
at the same time substantially less than the closest succeeding
higher integer number of symbol time intervals. If then desiring to
cope with the entire time dispersion by an Viterbi equalizer it is
necessary to design the equalizer for the biggest one of the two
integer numbers of symbol duration. The invention aims at solving
this problem in a mobile radio system having both analog and
digital radio channels.
Somewhat simplified one could say that the invention is based upon
the idea to use the lower density of the analog radio channels for
reflections and multipath propagation causing dispersion on the
digital radio channels. The invention is also based on the
understanding that the time dispersion on a particular radio
channel in most cases is substantially less than the maximum time
dispersion the system is to be designed for. Somewhat simplified it
could the be said that according to the invention the equalizers
are designed for a lower maximum time dispersion corresponding to
the lower of the integer numbers of symbol time intervals and
instead at least a temporary change over from a digital radio
channel to an analog radio channel is done when the time dispersion
on the used digital radio channel becomes greater than the maximum
time dispersion the equalizer is designed for. Accordingly handover
is done to an available analog radio channel. If there should be
another available digital radio channel being sufficiently good the
first choice is to attempt to use this by handover to such a
digital radio channel. In such cases handover to an available
analog radio channel is done only when there is no sufficiently
good available digital radio channel.
It is possible but not always self evident to return to a digital
radio channel by handover as soon as there is an available digital
radio channel being sufficiently good.
In order to get to know in time when the dispersion on the used
digital radio channel tends to become too severe the radio
propagation properties of the used digital radio channel ought to
be estimated more or less continuously. The estimation is
preferably done by estimation of signal level and symbol error
frequency, which can be performed in ways known per see. One method
has been presented by Italtel within the framework on the pan
european GSM system and is disclosed in a GSM paper titled GSM/WP2
doc 17/88. Based on the method suggested by Italtel Ericsson has
suggested a method of measuring and transferring bit error rate
information for the future U.S. digital cellular standard in a
paper forwarded to the TIA Technical Subcommittee, TR-45 Digital
Cellular Standards, meeting on Aug. 29, 1989 in San Diego,
Calif.
The estimation of signal level and symbol error frequency is
preferably performed at the mobile station which reports the
estimation to its base station. This is in accordance both with the
GSM standard and the TIA on digital cellular mobile radio
systems.
Instead of estimating time dispersion indirectly by estimation of
bit error rate and signal level the time dispersion may be
estimated directly using receiver equalizer and synchronizing words
transmitted on digital radio channels. Such direct estimation may
use methods based on the contribution to the 38th IEEE vehicular
technology conference held in Philadelphia, Pa., USA on 15-17 Jun.,
1988, titled "Bit synchronization and timing sensitivity in
adaptive viterbi equalizers for narrowband-TDMA digital mobile
radio systems", by A. Baier, G. Heinrich, U. Wellens.
When possible it is preferred to perform a more or less continuous
estimation also for at least on other digital radio channel than
the channel presently used by the mobile. At least the radio
propagation properties of at least one other possible digital radio
channel shall be performed when the estimation of the radio
propagation properties of the used digital radio channel indicates
that the used digital radio channel is or tends to be too poor.
In order to increase the likelihood of successful handover to an
analog radio channel the properties of the intended analog radio
channel also ought to be estimated in an appropriate way, e.g. the
expected signal level and/or the signal to noise ratio of the
channel may be estimated. When this is difficult to accomplish at
the mobiles it may be done at the base stations.
When introducing digital radio channels into a known mobile radio
system with analog radio channels where decisions on handoff are
done in the fixed part of the system and estimation of dispersion
and other radio signal properties may also be done in the fixed
part of the system the normal known handoff signalling procedure of
the system for analog radio channels may be used. In a mobile radio
system with a mobile assisted handoff procedure where at least some
measurements or estimations are done at the mobile the procedure
for handoff from a digital radio channel to an analog radio channel
due to dispersion must include some signalling of results of
measurements or estimations from mobile to base station.
THE DRAWINGS
FIG. 1 illustrates part of a cellular mobile radio system with
cells, a mobile switching center, base stations and mobile
stations.
FIG. 2 illustrates a plurality of radio channels within a frequency
band and use of some radio channels in a cellular mobile radio
system according to FIG. 1.
FIG. 3 illustrates use of radio channels according to FIG. 2 for
control channels, analog communication channels and time division
multiplex digital communication channels in a cellular mobile radio
system according to FIG. 1.
FIG. 4 illustrates bursts separated by guard spaces on a radio
channel used for digital communication channels in time division
multiplex according to FIG. 3 in a cellular mobile radio system
according to FIG. 1.
FIG. 5 illustrates a base station in a cellular mobile radio system
according to FIG. 1 with radio channels used according to FIGS. 2
to 4.
FIG. 6 illustrates a mobile station in a cellular mobile radio
system according to FIG. 1 for communication with a base station
according to FIG. 5 on control and digital communication channels
according to FIGS. 2 to 4.
FIG. 7 illustrates actual error rate and estimation of error rate
on a digital channel by using additional channel coder at the
receiving side of a base or mobile station.
FIG. 8 illustrates impulse response of digital channel radio link
including transmission and receiving means and estimation of time
dispersion on a digital channel using synchronization words.
DETAILED DESCRIPTION OF EMBODIMENTS
FIG. 1 illustrates ten cells C1 to C10 in a cellular mobile radio
system. Normally a method according to the invention is implemented
in a cellular mobile radio system comprising many more cells than
ten. However, for the purpose of explaining the invention ten cells
may be sufficient.
For each cell C1 to C10 there is a base station B1 to B10 with the
same number as the cell. FIG. 1 illustrates base stations in the
vicinity of cell center and having omnidirectional antennas. The
base stations of adjacent cells may however be colocated in the
vicinity of cell borders and have directional antennas as is well
known to those skilled in the art.
FIG. 1 also illustrates ten mobile stations M1 to M10 within a cell
and from one cell to another cell. Normally a method according to
the invention is implemented in a cellular mobile radio systems
comprising many more mobile stations than ten. However for the
purpose of explaining the invention ten mobile stations may be
sufficient.
Also illustrated in FIG. 1 is a mobile switching center MSC. The
mobile switching center illustrated in FIG. 1 is connected to all
ten illustrated base stations by cables. The mobile switching
center is connected by cables also to a fixed public switching
telephone network or similar network with ISDN facilities. All
cables from the mobile switching center to base stations and cables
to the fixed network are not illustrated.
In addition to the mobile switching center illustrated there may be
another mobile switching center connected by cables to other base
stations than those illustrated in FIG. 1. Instead of cables other
means may be used for base to mobile switching center
communication, e.g. fixed radio links.
The cellular mobile radio system illustrated in FIG. 1 comprises a
plurality of radio channels for communication. The system is
designated both for analog information, e.g. speech, digitized
analog information, e.g. digitized speech, and pure digital
information, e.g. pure data. In this application the term
connection is used for a communication channel between a mobile
station and another mobile station in the same system or another
system or a fixed telephone or terminal in a fixed network
connected to the cellular mobile radio system. Thus a connection
may be a call where two persons talk to each other but may also be
a data communication channel where computers exchange data.
FIG. 2 illustrates somewhat simplified a plurality of radio
channels RCH1 to RCH2n within a frequency band. A first group of
radio channels RCH1 to RCHN are used in the cellular mobile radio
system for transmission of radio signals from base stations to
mobile stations. A second group of radio channels RCHN+1 to RCHN2
are used in the cellular mobile radio system for transmission of
radio signals from mobile stations to base stations.
Some of the radio channels are used for control channels. Normally
each base station has at least one control channel. Normally a
control channel is not used for transfer of information on a
connection but for monitoring and control of mobiles during setup
of a connection, maintaining as established connection and handoff
of an established connection. In FIG. 3 is illustrated how the
radio channel RCHf all the time is used for a control channel CCHk
while the radio channel RCHg all the time is used for a control
channel CCHr.
Some of the radio channels are used for analog communication
channels. Normally analog communication are used for connections
where analog information is exchanged, e.g. telephone calls where
two persons talk to each other. Normally one analog communication
channel is required for each such connection. When a radio channel
is used for an analog communication channel the information on the
connection is transmitted with analog modulated radio signals. In
addition to the information on the connection the analog
communication channel may also be used for associated information,
e.g. a supervisory audio tone. In FIG. 3 is illustrated how the
radio channel RCHa all the time is used for an analog communication
channel ACHi while the radio channel RCHb all the time is used for
an analog communication channel ACHv. Normally each base station
has at least one radio channel used for analog communication
channel.
Some of the radio channels are used for digital communication
channels. Normally digital communication channels are used for
connections where digital or digitized information is exchanged,
e.g. data or digitized speech. The radio channels used for digital
communication channels are divided into time slots and the time
slots are grouped in frames. The time slots are allotted to digital
communication channels whereby multiple digital channels share a
common radio channel in time division multiplex. In FIG. 3 is
illustrated a radio channel RCHc having three time slots in each
frame F. A first of the time slots is allotted to the digital
communication channel DCH4, a second of the time slots is allotted
to the digital communication DCH5 and the third of the time slots
is allotted to the digital communication channel DCH6. Thus the
radio channel RCHc is used for three digital communication
channels. FIG. 3 also illustrates how the radio channel RCHd is
used for three digital communication channels DCH7, DCH8 and DCH9
in a corresponding way.
In FIG. 3 the frames F of radio channels RCHc and RCHd have three
time slots. Depending on the required bandwidth of the various
digital communication channels it is conceivable to have less slots
in a frame, e.g. two slots, or to have more slots in frame, e.g.
six slots. When the digital communication channels are used for
connections where digitized speech is exchanged, six time slots may
give too poor speech quality when the radio bandwidth is 30
kHz.
On a radio channel used for digital communication channels the base
or mobile station transmits a time slot identifier code with the
radio signals at least in every time slot used for a connection. On
a particular radio channel, e.g. RCHc, the time slot identifier
codes in different time slots are different. Thus the time slots
identifier code T11 is transmitted in the first time slot of radio
channel RCHc allotted to digital communication channel DCH4. The
time slot identifier code T12 is transmitted in the second time
slot of radio channel RCHc allotted to digital communication
channel DCH5. The time slot identifier code T13 is transmitted in
the third time slot assigned to digital communication channel
DCH6.
The same time slot identifier codes may be used on two or more
radio channels, possibly all radio channels. FIG. 3 illustrates how
the time slot identifier T11 is transmitted in the first time slot
of radio channel RCHd allotted to digital communication channel
DCH7. The time slot identifier code T12 is transmitted in the
second time slot of radio channel RCHd allotted to digital
communication channel DCH8. The time slot identifier code T13 is
transmitted in the third time slot of radio channel RCHd allotted
to digital communication channel DCH9. Thus the time slot
identifier code does not alone identify the channel but identifies
the time slot in a frame. It is conceivable to have one set of time
slot identifiers T11 to T13 for channels with three slot frames F
and have a different set of time slot identifiers T14 to T9 for six
slot frames whereby the time slot identifier also may indicate the
number of slots in a frame of the radio channel.
On a radio channel used for digital radio channels also transmitted
a digital voice color code with the radio signals at least in each
time slot used for a connection. On a particular radio channel the
same digital voice color code is transmitted with the radio signals
in different time slots. FIG. 3 illustrates the transmission of the
same digital voice color code VC1 in all time slots of the radio
channel RCHc. Normally the same digital voice color code is used
for all radio channels to and from a particular base station, e.g.
the digital voice color code VC1 is used for all radio channels to
and from base station BS1.
Some adjacent base stations may use the same digital voice color
code e.g. base stations B2, B6 and B7 may use the same digital
voice color code as long as base station BS1. Other base stations
use a different digital voice color code, e.g. base stations B4,
B5, B9 and B10 all use the digital color code VC4. Still other base
stations use another different digital voice color code, e.g. base
stations B3 and B8 use the digital voice color code VC7.
The radio signals on a radio channel used for time division
multiplex digital communication channels are transmitted in bursts.
FIG. 4 illustrates a burst in a time slot separated by guard spaces
from the end of the preceding burst and the beginning of a
succeeding burst in adjacent time slots.
A transmitted burst comprises at least a time slot identifier,
abbreviated T1, and a digital voice color code, abbreviated VC, and
normally also information to be forwarded to the other party of the
connection as well as connection or channel associated information
for control or monitoring purposes.
It is well known to those skilled in the art that there is a need
for receiver synchronization in time division multiple access radio
communication systems. For this purpose it is well known to
transmit synchronization words or patterns in each burst and to
transmit particular frame synchronization words or patterns from a
master or base station to a slave or mobile station. Preferably the
time slot identifier code according to the present invention may
also be used for synchronization of receiver to transmitter.
Selecting time slot identifier codes in this way means that the
implementation of time slot identifier codes according to the
present invention does not require any dedicated bits for the time
slot identifier. Theoretically, there exist a number of
uncorrelated binary multi bit words, e.g. 26 bit words, which may
be used as different synchronization words. According to the
present invention one unique time slot identifier and
synchronization words is required for each time slot in a frame on
the channel. For the purpose of both time slot identification and
synchronization the time slot identifier code words are to be
chosen for minimum correlation between each other and between
themselves except for correlation to themselves in phase. Thus a
first time slot identifier code used to identify the first time
slot of a frame according to the present invention and to
synchronize receiver to transmitter shall exhibit low correlation
to an identical code when substantially out of phase but exhibit
high correlation to an identical code when substantially in phase.
A second time slot identifier code used to identify the second time
slot of a frame and to synchronize receiver to transmitter shall
also be exhibit low correlation to an identical code when out of
phase but exhibit high correlation to an identical code when
substantially in phase. All other time slot identifiers used on the
channel should also exhibit low correlation to an identical code
when out of phase but exhibit high correlation to an identical code
when in phase. Any time slot identifier code used on the channel
should also exhibit low correlation to any other time slot
identifier code used on the channel irrespective of phase
relationship, i.e. of in phase or out of phase. Once the
requirements on time slot identifier codes are given one skilled in
the art will be able to select proper time identifier codes without
inventive activity. However, for the convenience of those not
skilled in the art the following eight 26 bit words are given as
examples of possible time slot identifier codes for an eight slot
frame:
code for time slot 1:
(0,0,1,0,0,1,0,1,1,1,0,0,0,0,1,0,0,0,1,0,0,1,0,1,1,1,)
code for time slot 2:
(0,0,1,0,1,1,0,1,1,1,0,1,1,1,1,0,0,0,1,0,1,1,0,1,1,1,)
code for time slot 3:
(0,1,0,0,0,0,1,1,1,0,1,1,1,0,1,0,0,1,0,0,0,0,1,1,1,0,)
code for time slot 4:
(0,1,0,0,0,1,1,1,1,0,1,1,0,1,0,0,0,1,0,0,0,1,1,1,1,0,)
code for time slot 5:
(0,0,0,1,1,0,1,0,1,1,1,0,0,1,0,0,0,0,0,1,1,0,1,0,1,1,)
code for time slot 6:
(0,1,0,0,1,1,1,0,1,0,1,1,0,0,0,0,0,1,0,0,1,1,1,0,1,0,)
code for time slot 7:
(1,0,1,0,0,1,1,1,1,1,0,1,1,0,0,0,1,0,1,0,0,1,1,1,1,1,)
code for time slot 8:
(1,1,1,0,1,1,1,1,0,0,0,1,0,0,1,0,1,1,1,0,1,1,1,1,0,0,)
When there are less than eight time slots in a frame on the radio
channel less than all of the given code words may be used. However
there may be advantages in using other than the given code when
there are only three time slots in a frame.
It of course conceivable to use binary time slot identifier codes
having more or less than 26 bits. Longer code words offer some
advantages but has the disadvantage of occupying more of the
available space in a burst.
In the advanced mobile phone service system, abbreviated AMPS,
there is a supervisory audio tone, abbreviated SAT, transmitted
e.g. on analog communication channels. The reason for transmitting
the SAT in AMPS is that in an interference limited mobile radio
communication network there should be some means for the receiving
entity (e.g. base station) to identify the transmitting entity
(e.g. mobile station) or at least with high likelihood exclude
interchange of transmitter entity without the need for continuous
transmission of transmitter identity. The object of the digital
channel code in a cellular mobile radio system according to the
present invention is at least partly the same as that of SAT in
AMPS. The number of different SATs is three in AMPS. The number of
different digital voice color codes in a cellular mobile radio
system according to the present invention is preferably much higher
than three, mainly to support a proper introduction of
discontinuous transmission. The number of different digital color
codes should be high enough to guarantee that cells where the same
radio channels are used having the same color code ar at a
geographical distance from each other sufficient not to cause
co-channel interference. In case discontinuous transmission is used
the disturbing signal of another station should not significantly
exceed the level of noise. For this purpose an eleven bit color
code seems quite sufficient in normal cases. However, the longer
color codes the longer connection setup and handoff procedures.
Considering the possible load on control channels eight bit digital
voice codes seems to be a good compromise. Preferably the digital
voice channel code is binary word having eight bits, enabling
theoretically 256 different voice color codes.
A separate digital voice color code requires space in a burst thus
reducing the space available for speech or data on the connection.
However, the following implementation of voice code does not
require any dedicated bits for the digital voice code in a burst on
a digital communication channel.
At the transmitting side the digital voice color code is added bit
by bit modulo two to a field under error detection but without
error correction within the information part of the burst after
channel coding. On the receiving side the bursts in the time slots
are deinterleaved and checked. This check is done by adding bit by
bit the known digital voice color code modulo two, as is done at
the transmitting side, before channel decoding and error detection.
If no error is found after adding the digital voice color code
modulo two the information part of the burst was sent from the
expected transmitter and not from an interferer.
FIG. 5 illustrates a base station in a cellular mobile radio system
according to FIG. 1 with radio channels according to FIGS. 2 to 4.
The base station is designed for transmission and reception on a
plurality of radio channels used for digital communication
channels, analog communication channels and control channels. In
FIG. 5 not all base station equipment for all channels is
illustrated. Normally a base station has equipment for more
channels, in particular analog communication channels, as well as
other equipment for power supply, maintenance etc but the
illustrated equipment is believed sufficient for the understanding
of the present invention.
The base station is connected to a mobile switching center by six
trunks. A first incoming trunk for digital communication channels
is connected to a digital trunk demultiplexer and interface DMU-D.
A second incoming trunk for analog communication channels is
connected to an analog trunk interface and demultiplexer DMU-A. A
third incoming trunk for control channels and base station control
information is connected to a trunk interface and control
information demultiplexer DMU-C. A first outgoing trunk for digital
communication channels is connected to a digital multiplexer and
trunk interface MUX-D. A second outgoing trunk for analog
communication channels is connected to an analog multiplexer and
trunk interface MUX-A. A third outgoing trunk for control channels
and base station information is connected to a control information
multiplexer and trunk interface MUX-C.
For each of the outgoing radio channels used for digital
communication channels the base station comprises digital analog
channel coding means, DCC, connected to a digital trunk
demultiplexer and interface, DMU-D, modulo two adding means, M2A,
burst generation means, BG, modulator means, MOD, and power
amplification means, PA, connected to an antenna. Two such outgoing
radio channels share a digital voice color code means, VCS,
connected to their modulo two adding means. Two such outgoing radio
channels also share a time slot identifier code means, TIS,
connected to their burst generators.
For each of the outgoing radio channels used for analog
communication channels the base station comprises analog
transmission channel processing means, ATC, connected to an analog
trunk interface and demultiplexer, DMU-A, modulator means, MOD, and
power amplification means, PA, connected to an antenna.
For each of the outgoing radio channels used for control channels
the base station comprises outgoing control channel processing
means, CTC, connected to the trunk interface and control
information demultiplexer, DMU-C, modulator means, MOD, and power
amplification means, PA, connected to an antenna.
For each incoming radio channel used for digital communication
channels the base station comprises radio receiver means, REC,
connected to an antenna, radio signal strength or level measuring
means, SLM, analog to digital conversion means, A/D, multipath
equalizer and burst synchronizing and time slot recognition and
automatic frequency control means, EQ-AFC, modulo two adding means,
M2A and digital channel decoder means, DCD, connected to a digital
multiplexer and trunk interface, MUX-D.
Two incoming radio channels used for digital communication share
digital voice color code means, VCS, connected to their modulo two
adding means. Two such incoming radio channels also share digital
channel bit error measuring means, BEM, connected to their digital
channel decoders, DCD.
For each incoming radio channel used for analog communication
channels the base station comprises radio receiver means, REC,
connected to an antenna, radio signal strength or level measuring
means, SLM, and incoming analog channel processing means, ARC,
connected to an analog multiplexer and trunk interface, MUX-A.
For each incoming radio channel used for control channel the base
station comprises radio receiver means, REC, connected to an
antenna, radio signal strength or level measuring means, SLM, and
incoming control channel processing means, CRC, connected to the
control information multiplexer and trunk interface, MUX-C.
All modulation means and radio receiver means are connected to
frequency synthesizer means, FS. The frequency synthesizer means
are controlled by a central processor, CP. The CP also controls the
DCCs, VCSs, BGs, EQ-AFCs, DCDs, BEM, ATC, ARC, CTC, CRC and MUX-C.
Preferably the central processor is not the only processor in the
base station but other means may also comprise processors, in
particular the ATC, ARC, CTC, CRC and EQ-AFCs.
The base station according to FIG. 5 is intended for communication
with mobile station only having equipment for analog communication
channels and control channels. The base station is also intended
for communication with mobile stations only having equipment for
digital communication channels and control channels. The base
station is also intended for communication with dual mode mobile
stations designed for communication on both analog and digital
communication channels as well as channels.
Mobile stations designed only for analog communication channels may
be of a kind well known to those skilled in the art and operate
according to AMPS standard. Furthermore, a method according to the
invention can only be implemented with mobile stations with means
for communication both on digital and on analog radio channels.
Accordingly there is no need for disclosing a mobile designed only
for analog radio channels or its operation here. Neither is there
any need for describing parts of a dual-mode mobile station used
only for communication on analog radio channels and control
channels.
FIG. 6 illustrates parts of a dual-mode mobile station in a
cellular radio system according to FIG. 1 for communication with a
base station according to FIG. 5 on radio channels according to
FIGS. 2 to 4. The parts illustrated are involved in communication
on digital radio channels. A dual mode mobile station capable of
using both analog and digital radio channels thus includes in
addition to the means illustrated in FIG. 6 first analog signal
processing means connected to the microphone and the modulator and
second analog signal processing means connected to the radio
receiver IF stage and to the loudspeaker. The first and second
analog signal processing means are merely indicated by one block
and are controlled by the microprocessor.
The mobile station comprises a microphone connected to analog to
digital speech coding means for coding speech or sound into a
binary code with a bitrate or bandwidth less than 11 kHz.
preferably about 7 to 8 kHz. Connected to the speech coding means
is channel coding means for interleaved error protecting coding of
the digital information from the speech coder. The channel coder is
connected to a modulo two adder for modulo adding of a digital
voice color code to the digital information from the channel coder.
The modulo two adding means are connected to a burst generator for
collecting information to be transmitted in a burst and arranging
the information including time slot identifier code into a proper
burst. When the mobile is used for data transmission on a
connection or during connection setup the burst generator puts data
and/or control information in the burst instead of digitized
speech. Such information may be supplied from a keyboard via a
microprocessor and the channel coder or directly from the
microprocessor. A modulator is connected to the burst generator for
receiving digital information to be modulated on a radio frequency
carrier from as frequency synthesizer and be amplified in a power
amplifier. The modulation method used may be continuous phase
modulation or other types suitable for digital information
transmission. The power amplifier is connected to an antenna via a
duplex and is controlled from the microprocessor.
The mobile station also comprises a radio receiver connected to the
duplexer, a radio signal or level means and analog to digital
conversion means. The radio receiver comprises RF and IF stages
with filter, demodulator etc. Means for equalizing the digital
communication channel and automatic frequency control and automatic
gain control are connected to the radio receiver and the input of a
modulo two adder. The modulo two adder adds a digital voice color
code modulo two to the digital information from the equalizer. The
modulo two adder output is connected to a channel decoder for
deinterleaving and error detection and correction of digital
information from the modulo two adder. Means for converting digital
information to analog information or speech are connected to the
channel decoder and a loudspeaker.
When the mobile station is tuned to a radio channel used by a base
station for control channel some of the illustrated equipment of
the mobile station is not used, in particular not channel and
speech decoder. When control and monitoring information is
transmitted from the base station on the control channel according
to the AMPS standard the microprocessor receives and interprets
signals from the analog to digital converter.
Except for the time slot identifier and the digital voice color
code and means for introducing, recognizing and removing them from
the flow of information the mobile radio according to FIG. 6 may
operate on time division multiplex digital communication channels
in a way similar to known digital mobile radio stations, e.g. of
the kind disclosed in the GSM standard or in Ericsson Review No. 3,
1987.
Accordingly there is no need here to further describe in detail the
overall operation or the detailed operation of the various means.
Regarding the time slot identifier code and the digital color code
the mobile station comprises means for storing all possible time
slot identifier codes and digital voice color codes to be used in
communication with a base station. The microprocessor receives
instructions from the base as to which codes to use in the
particular connection and reads the codes from the stores and
supplies the modulo two adders and burst generator with appropriate
time slot identifier code and digital voice color code.
When receiving radio signals from a base station synchronization
and recognition of time slot identifier is performed in the
equalizer in cooperation with the microprocessor. Measurement of
bit error rate on an established connection is performed in the
channel decoder in cooperation with the microprocessor. Equalizers
and methods of synchronization and bit error measurement are well
known to those skilled in the art. Accordingly there is no need
here to further describe such methods or means for performing them.
However for the person not skilled in the art a brief explanation
on bit error measurement for digital radio channels will be given
in connection with FIG. 6 and 7.
In this example it is assumed that at least a part of the digital
information transmitted on the digital radio channel is protected
by an error correction code. As an example, both the GSM and TIA
systems use 20 ms speech blocks with part of the speech coder
output bits protected by an error correction code. Since the speech
coder operates block wise so does the error correction applied. The
actual bit error rate, BER, of the radio channel including certain
transmitting and receiving means at base and mobile station is of
course due to the difference between the information actually
leaving the channel coder of the transmitter and the information
received by the receiver decoder. This is illustrated in the left
hand part of FIG. 7. An estimate of the actual bit error rate can
be done by reencoding the decoded data in the receiver and
comparing this bit stream with the input to the channel decoder of
the receiver. This is illustrated in a principal way in the right
hand part of FIG. 7. For the purpose of this an additional channel
encoder may be used and in FIG. 6 an additional encoder has been
incorporated in the receiving part of the mobile station.
Corresponding additional encoder means may of course be
incorporated in the receiving part of a base station according to
FIG. 5. For comparing corresponding digital symbols according to
FIG. 7 the central processor in the base station and the
microprocessor of the mobile station may be used.
If the channel decoder in the receiver has corrected all bit errors
in a block of n bits then the bit by bit comparison of the received
data with the reencoded data will be equal to the actual number of
bit errors in the block of n bits. The number of bit errors is the
number of different bit values in the above mentioned comparison.
Thus, in this case the estimated number of bit errors is equal to
the true value of bit errors. The bit error rate in the block of n
bits is then the number of bit differences divided by n.
If the channel decoder of the receiver is not capable of
reproducing the transmitted n bits e.g. the channel decoder makes
an error, the estimated bit error rate will not be equal to the
actual error rate. Nevertheless, for bit error rates in the range
of interest the estimate will be highly correlated to the actual
rate. The difference between the estimated and actual rate can be
seen as a measurement noise term. The system is intended to operate
with a radio link quality where most of the blocks can be corrected
by the channel decoder, otherwise the audio quality would be
unintelligible, thus the measurement noise is limited. If the radio
link quality is so low that almost every block is uncorrectable by
the channel decoder, the output from the decoder is more or less a
random pattern with low correspondence with the received block.
This can for instant happen if the amount of time dispersion is
much more than the equalizer can handle. The important thing is
that the bit error estimate will still cause a generally correct
conclusion, i.e. too low radio channel quality, because the
difference between two blocks with almost no correspondence will
result in that, in average, every second bit comparison will
indicate a bit error. Thus the bit error rate will be estimated to
be 50%.
In summary, independent of the radio link quality this method
described will indicate to the receiver the quality of the radio
link in terms of a bit error estimate.
The procedure for connection setup in a cellular mobile radio
system according to FIG. 1 with a base station according to FIG. 5
and a mobile station according to FIG. 6 may be similar to the
corresponding procedure in AMPS when the channel to be used is an
analog communication channel. However, when the channel to be used
for the connection is a digital communication channel according to
FIGS. 3 and 4 the base station informs the mobile station not only
on radio channel but also on time slot and digital voice color code
to be used. During the setup procedure the base station then also
informs the mobile station on a plurality of radio channels the
signal strength on which to be measured by the mobile. Normally
this plurality of radio channels are the radio channels used for
channels by adjacent bases/cells. Depending upon the movement of
the mobiles as well as other circumstances a new plurality of radio
channels may be selected and corresponding information be
transmitted to the mobile from the responsible base station during
the course of the connection. During the course of a connection for
which a digital communication channel is used the mobile measures
the signal strength of signals on the given plurality of radio
channels. Measurements may be done during time slots not used by
the digital communication channel. The mobile station also measures
signal strength on the digital communication channel used for the
established connection and the bit error rate on the established
connection. The mobile station transmits results of its
measurements, preferably averaged, frequently to the base station,
preferably twice a second.
In addition to or instead of measuring signal strength and bit
error rate the mobile station may estimate time dispersion on the
used digital radio channel using the time slot identifier code
words and the equalizer in a way to be described later. The mobile
station may transmit the estimation in addition to or instead of
signal level or bit error rate measurement results frequently to
the base station.
The base station also measures signal strength on the digital
communication channel used for established connection and the bit
error rate on the established connection. The base station may also
estimate time dispersion on incoming radio channels used for
digital communications channels. This may be done using the time
slot identifier codes and equalizer in a way to be described later.
The base station processes and analyzes the results of its own
measurements and/or time dispersion estimates and the measurement
and/or time dispersion estimates of the mobile and compares with
criteria for handoff. When according to the results and criteria a
handoff to another base station is desirous the base station
informs the mobile switching center indicating at least one target
base station assumed suitable for taking over the responsibility
for the communication with the mobile.
The mobile switching center request the target base station(s) to
measure signal strength on a radio channel in a time slot used by
the mobile for the established connection. The mobile switching
center also informs the target base station on the digital color
code used by the mobile station.
The target base station(s) tune (s) a receiver to the radio channel
indicated by the mobile switching center and uses the time slot
identifier of the indicated time slot for burst synchronization.
The target base station checks the appearance of the digital color
code indicated by the mobile switching center and measures the
signal strength of the burst signals provided the digital color
code is correct. The target base station then transmits its results
of signal strength measurement to the mobile switching center. The
target base station also informs the mobile switching center on the
result of the checking of the appearance of the digital color code,
that is whether the digital voice color code appeared in the burst
in the time slot of the radio channel.
The mobile switching center determines whether handoff to a target
base should be performed taking the results of signal strength
measurements of target base(s) into account as well as other
circumstances, e.g. traffic load.
When the mobile switching center determines that handoff to an
other base station and digital radio channel shall be performed it
transmits to responsible base and target information on new radio
channel, new time slot and new voice color code to be used by the
mobile station for the connection after handoff and new radio
channel to be used by target base station for the connection after
handoff.
The responsible base station forwards information about the two new
radio channels, new time slot and new digital color code to the
mobile. After receiving this information the mobile station tunes
to the new radio channel to be used for the connection by the
target base station and looks for the new time slot identifier code
in received signals on the radio channel. The mobile station uses
the new time slot identifier code in received signals for burst
synchronization. After synchronization and tuning its transmitter
to the new radio channel the mobile station begins transmitting
bursts in the new time slot on the new radio channel. The new
digital color code is transmitted with each burst.
The target base station tunes a receiver to the new channel to be
used for the connection by the mobile station and looks for the new
time slot identifier code. The target base station uses the time
slot identifier code for synchronization. The target base station
then looks for the new digital color code in signals in the new
time slot of the new channel. If the target base station identifies
the new digital color code in the bursts in the new time slot of
the new radio channel this is reported to the mobile switching
center. The mobile switching center then interprets the handoff as
successful and acts accordingly. After successful handoff the
former target base station now being responsible base station
informs the particular mobile station on a new plurality of radio
channels the signal strength on which to be measured by the
mobile.
In the embodiment of the handoff method described above the
responsible base station and mobile station use the same time slot
identifier code and the same digital voice color code. However, it
is conceivable to use different time slot identifier at base and
mobile stations for a particular connection.
In the embodiment of the handoff method described the mobile
station measures signal strength on radio channels used for control
by base station. However, it is conceivable to request mobile
station to measure signal strength on radio channels used for
digital communication channels by base station, in particular when
there are no radio channels entirely used for control channels by
base stations.
The procedure for intracell handoff from a digital radio channel to
an analog radio channel of the same base station is of course less
complex than the described handoff procedure because responsible
base station and target base station is the same. When according to
the results and criteria a handoff from a digital radio channel to
an analog radio channel is desirous the base station informs the
mobile switching center. After approval by the mobile switching
center the base station may perform the normal procedure of the
system for intracell handoff to a new analog radio channel, the
procedure comprising transmitting informing to the mobile station
on the new radio channel to be used. If the invention is
implemented in a mobile radio system where handoff between analog
channels is performed according to the AMPS standard the this
procedure may also be used for handoff from digital to analog
handoff due to time dispersion. If the invention is implemented in
a mobile radio system where handoff between analog channels is
performed according to an other standard then this procedure may
also be used for handoff from digital to analog channel due to time
dispersion. Accordingly there is no need for describing a complete
such a procedure here. Suffice it to say that there is no
transmission of information on time slot identifier code or digital
voice color codes from base to mobile but instead information on
supervisory audio tones or similar means may be transmitted from
base to mobile.
The procedure for handoff from a digital channel of one base
station to an analog channel of an other base station is of course
a little more complex than the intracell handoff procedure because
more than one base station is involved. However, the normal
procedure of the system for handoff to a new base station and a new
analogue channel may be used, the procedure comprising transmitting
information to the mobile station on new base station and new
analog channel. If the invention is implemented in a mobile radio
system with analog channel handover according to the AMPS standard
then this procedure may be used. If the invention is implemented in
a system with different handover procedure then this handoff
procedure is well known to those skilled in the art. Anyone not
skilled in the art of procedures for analog channel handover is
recommended to study the specifications on the NMT and AMPS systems
or the specification of the system where the invention is to be
implemented and possibly also published patents in the field of
handover. Accordingly there is no need for describing such a
handover procedure here.
The intelligence and decision making in a mobile cellular system
according to FIG. 1 may be more or less centralized to the mobile
switching center or more or less decentralized to the base
stations. In a decentralized system more or less of the functions
of the mobile switching office during the handoff preparation and
performance may instead be performed in the responsible and/or
target base stations.
A method of directly estimating time dispersion and determining
when the time dispersion is too big will now be described in
connection with FIGS. 6 and 8.
The signals from the A/D converter in FIG. 6 are sampled a number,
j, of times per bit time. In the equalizer the received signals are
correlated with the locally stored time slot identifier. In this
way an estimate Y(t) of the channel impulse response is calculated
in the equalizer and made available for processing and equalization
in the central processor. In FIG. 8 the absolute values of Y(t) are
indicated as a function of time. The distance between adjacent
indicated values of Y(t) is the symbol (bit) time, T, divided by j.
A process of estimating the impulse response is described in detail
in the earlier cited article "Bit synchronization and timing
sensitivity in adaptive Viterbi Equalizers for Narrowband-TDMA
Mobile Radio Systems". Accordingly for a further understanding of
FIG. 8 reference is made to this article.
The equalizer correlates the received signal with the time slot
identifier TSID over a movable time span Wt, hereafter called the
measurement window, which is indicated in FIG. 8. The length of the
measurement window represent the amount of time dispersion that the
equalizer can handle e.g. energy within the measurement window
contributes in a positive manner for the demodulation/bit detection
process whereas energy outside this window act as an interfering
signal. The length of the gliding window is a design parameter in
the construction of the equalizer. With a long window the equalizer
can handle more time dispersion but the complexity and power
consumption will increase.
The equalizer moves the gliding window in relation to the impulse
response, calculates the impulse energy inside and places the
window in such a position in relation to the impulse response that
the part of the energy, c, inside the window is maximized. This
part of the impulse inside the window is shadowed in FIG. 8.
Maximizing the part of the energy inside means that the impulse
energy, r, outside the measurement window is minimized. The
positioning of the movable window is further elaborated in the
above mentioned reference. The estimation of the impulse response
is an inherent feature of the equalizer. Depending on the
equalization algorithm used, the impulse response may be estimated
only once in each burst, as described above, but may also be
updated during the duration time of the burst. Thus, the central
processor may obtain at least once every burst the amount of useful
energy, c, and the interfering energy, r, from the equalizer.
The receiver processor can now average the individual c and r
values over a plurality, n, of bursts so that influence of
estimation errors, short time variations of radio propagation
properties etc is reduced. The averaged quantities of c and r are
denoted C and R respectively. Since C represents useful energy and
R represents disturbing energy the C/R ratio thus represents a
figure of merit of the ability of the equalization to reproduce the
transmitted data in a way similar to the well known S/N ratio used
for telecommunication in general.
If the C/R ratio is low the bit error rate will be high. An
criteria that can be used to determine if the time dispersion is
too severe to enable reproduction of an audio signal or data
transmitted is to make the processor comparing the C/R with a
threshold, K. Thus if C/R is less than K, the processor has
determined that the time dispersion is so big that problem with the
time dispersion currently exists.
Of course the described method of estimation of time dispersion on
a digital radio channel can be done either at the base station or
at the mobile station or at both. The invention is not limited to
the use of this particular method but decisions to handover to an
other channel may be based on other methods of estimation of time
dispersion.
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